Noncoordinate transcription of the M-MSV genome was observed in revertants from S plus L-Moloney-3T3FL cells. M-MSV specific nucleotide sequences were lost from the cellular RNAs of these revertants but common sequences of M-MSV which contained at least part of the gag gene were observed. Portions of transcribed common sequences, however, varied in degree of hybridization and they were complementary. A focus assay for simian sarcoma associated virus was established in rabbit cornea cells (SIRC). Proviruses of ml and HT-l MSV were cloned in a bacteriophage lambda vector. The lambda-MSV hybrids contained the entire viral genomes with host flanking sequences, demonstrated terminal redundant sequences, and were transfectable.